Near-infrared spectroscopy has been used for non-invasive measurement of various physiological properties in animal and human subjects. The basic principle underlying the near-infrared spectroscopy is that a physiological medium, such as a tissue, includes a variety of light-absorbing chromophores and light-scattering substances which can interact with low energy near-infrared photons transmitted thereto and traveling therethrough. For example, deoxygenated and oxygenated hemoglobins in human blood are the most dominant chromophores in the spectrum range of 700 nm to 900 nm. Therefore, the near-infrared optical spectroscopy has been applied to non-invasively measure oxygen levels in the physiological medium in terms of tissue hemoglobin oxygen saturation (or simply oxygen saturation hereafter). Technical background for the near-infrared spectroscopy and diffuse optical imaging has been discussed in, e.g., Neuman, M. R., “Pulse Oximetry: Physical Principles, Technical Realization and Present Limitations,” Adv. Exp. Med. Biol., 220:135-144, (1987), and Severinghaus, J. W., “History and Recent Developments in Pulse Oximetry,” Scan. J. Clin. And Lab. Investigations, 53:105-111, (1993).
Various techniques have been developed for the non-invasive near-infrared spectroscopy, including time-resolved spectroscopy (TRS), phase modulation spectroscopy (PMS), and continuous wave spectroscopy (CWS). (Chance, U.S. Pat. No. 5,553,614; Chance, U.S. Pat. No. 6,246,892; and Tsuchiya, U.S. Pat. No. 5,477,051). The TRS and PMS are generally used to solve the photon diffusion equation, to obtain the spectra of absorption coefficients and reduced scattering coefficients of the physiological medium, and to estimate concentrations of the oxygenated and deoxygenated hemoglobins and oxygen saturation. To the contrary, the CWS method cannot distinguish the light scattering and absorption properties and has generally been used to calculate relative values of or changes in the concentrations of the oxygenated or deoxygenated hemoglobins. (Cheng et al., U.S. Pat. No. 6,516,209; Cheng et al., U.S. Pat. No. 6,597,931).
PMS has been known as the most cost effective method to quantify the scattering and absorption properties of turbid medium compared to TRS. Various methods have been developed to calculate the absorption and scattering properties. Most methods have been using both the amplitude and phase measurement and using photon diffusion equations. However, there are problems involving in these methods. First, they are usually very noisy since the amplitude measurement is affected by many elements, such as source, detector drift, sensor attachment, etc. Second, there is no analytical inverse solution for the diffusion equation which results in it being hard to conduct accurate calculation. Finally, despite their capability of providing averaged quantitative hemoglobin concentrations and the oxygen saturation, the general problem of the TRS and PMS is that it is difficult to quantify concentrations of chromophores of a localized position inside tissues, such as the cortical tissue inside the head. (Cheng et al., U.S. Pat. No. 6,516,209; Cheng et al., U.S. Pat. No. 6,597,931). However, such localized information is usually critical for clinical and medical applications, such as cortex perfusion monitoring, diagnosis of hematoma, stroke, organ function monitoring, etc.
Various imaging approaches have been explored for imaging biological tissues using near-infrared photons. Nevertheless, there is no existing method which worked effectively, especially for the applications where only back reflected photons are detectable and transmitted photons are too weak to be detected, such as measurement on head or chest. The major challenge has been due to the scattering effect. Photons quickly lose their original travel direction and become randomized, and photon density exponentially decreases as photons travel deeper into the tissue. As a result, the detected photons always come from a volume of tissue, and the majority of which always comes from the superficial part of the tissue. Accordingly, in order to obtain properties of a local region inside tissue, one needs to deconvolve the contribution of all local region tissue to the signal.